Enabling OpenCL GPU rendering for Cycles on Linux?
To enable opencl on linux working with blender, the only way is to install the official amd-pro driver, or, as I did, only the opencl part.
Unfortunately, the only Linux distro officially supported are:
RHEL 8.0 / CentOS 8.0 and 7
Ubuntu 18.04.3 (and derivative) (so, all the kubuntu, xubuntu, lubuntu etc and mint)
SLED/SLES 15 SP1 (and opensuse leap)

So, with Debian You are in a bad spot, unless You play around quite a bit